The assembly of small parts, such as lenses applied to fiber optic applications and flex suspension assemblies used in hard disk drives, frequently calls for the dispensation of nanoliter or sub-nanoliter volumes of applicable adhesive in precise amounts and locations. The two common methods of dispensing small quantities of liquid adhesives are based on (1) a “time flow” controller or (2) “positive displacement” by precision movement of a piston or auger. A time flow controller typically involves a valve being pneumatically opened for a precise time period to produce the flow of adhesive from a reservoir through a needle or dispensing tip. The volume extruded is a function of the amount of time the valve is open, the amount of pressure on the fluid, the viscosity of the fluid and the size of the dispensing tip. One problem with the time flow and positive displacement devices is the requirement for extremely accurate and repeatable “Z” axis (the space between the dispensing needle and the target substrate) positioning and control. In addition, these devices have somewhat limited use, because they are generally designed to require gravity assistance and thus require downward dispensation.
Another type of dispensing device generally involves an action similar to an ink-jet printer. The device operates by shooting small drops of adhesive onto the target substrate. When a very small dot of adhesive is required, the device shoots only one drop. However, if a larger dot is desired, the device shoots more than one drop in the same location on the target substrate. The “ink-jet” style of dispenser is nearly omnidirectional in operational position relative to the target. They are limited, however, to dispensing fluids with a viscosity of less than about 100 cps. The range of application (single drop) is about 1 pL to about 1 nL.
Another conventional method comprises a plurality of pins that are mechanically fixed to a plate in a pattern that matches the location of desired application. The pin ends are dipped in a pool of adhesive, which adheres to the ends. The plate with the “primed” pins is then positioned over the target substrate so that the adhesive on the pin ends is placed in contact with the substrate. Adhesive is transferred from the end of the pin to the substrate. The quality of the dot depends on the material, pin diameters, dwell time, and offset height. This process as applied in early embodiments worked well for building the large printed circuit boards used in early electronic devices. However, engineering the fixed pin plates for the tiny components required today would be extremely difficult and expensive. In addition, these fixtures were customized for a specific application which limited their use in modern “flexible” manufacturing.
Accordingly, a need exists for a liquid dispensing device which reduces the number of variables in determining the amount of dispensed liquid and easily controls the remaining variables, while remaining capable of dispensing extremely small, precisely reproducible volumes of liquids. That is, a need exists for a liquid dispensing device capable of dispensing extremely small, precisely reproducible volumes of liquids in precise locations without requiring extremely precise control of the position of the device with relation to the substrate in the Z-axis. A need also exists for a liquid dispensing device which is not limited to dispensation of fixed incremental amounts of liquid, but rather is capable of dispensing any amount along a dispensation continuum. A further need exists for a liquid dispensing device which is capable of dispensing extremely small, precisely reproducible volumes of liquids in any direction, not just downward.